Understanding and practically applying the knowledge of how and when to harness the routing features of linux is a matter of experience. The below is a set of examples for how to use the RPDB and multiple routing tables to solve different types of problems. These are but a few simple examples which allude to the flexibility and power available with the complex policy routing system under linux.
Type of Service (ToS) is a flag in the header of an IP packet which is sometimes honored by upstream routers. Some routers on the Internet respect the ToS flag and others do not, however, the ToS flag can be used as part of the decision about where to route a given packet (for a refresher on the keys used for routing to a destination read Section 4.5, “Route Selection”). Because it can be used as part of the routing decision, ToS can be used to select a route separate from the route chosen for normal packets (packets not marked with any ToS).
FIXME!! Don't forget to point out that fwmark with ipchains/iptables is a decimal number, but that iproute2 uses hexadecimal number. Thanks to Jose Luis Domingo Lopez for his post to the LARTC list!